1. Field of the Invention
The invention relates to optical disk drives, and more particularly to automatic power control for optical disc drives.
2. Description of the Related Art
A pickup head of an optical disk drive projects a laserbeam onto a data layer of an optical disk to write data thereto or read data therefrom. When the pickup head emits the laserbeam with a high power level, the laserbeam melts the data layer of the optical disk or changes the phase of the data layer, scarfing data patterns thereon, thereby recording data onto the optical disk. When the pickup head emits the laserbeam with a low power level which can not melt/change the data layer of the optical disk, the pickup head decodes data according to the pattern of the reflection of the laserbeam from the disk, thereby reading data from the optical disk. A pickup head of an optical disk therefore must be capable of generating laserbeams with different power levels corresponding to the different functions of the optical disk drive.
A pickup head generates a laserbeam with a laser diode. The power level of the laserbeam emitted by the laser diode is controlled by a driving current. When the pickup head continues to generate the laserbeam, the laserbeam gives off heat which increases the temperature of the pickup head. Along with increasing temperature of the pickup head, driving current must be increased, thus controlling the laser diode to emit the laserbeam with a constant power level. Referring to FIG. 1, a schematic diagram of a relationship between a driving current level and a laserbeam power level is shown. When the pickup head is operated in a temperature T1, the relationship between the driving current level and the laserbeam power level is depicted by a line L0. When the temperature of the pickup head is changed to T2, the relationship between the driving current level and the laserbeam power level is changed to lines L1 or L2. The lines L0 and L1 have different offset levels Ith(T1) and Ith(T2) for generating a laserbeam with a minimum power level, and the lines L0 and L2 have different slopes s(T1) and s(T2).
To prevent the power level of the laserbeam from decreasing when the temperature of the pickup head increases, the optical disk drive must comprise an automatic power control mechanism to adjust the driving current of the laser diode according to the temperature, thus maintaining the laserbeam at a constant power level. Referring to FIG. 2, a schematic diagram of signals generated by a conventional automatic power control mechanism is shown. The conventional automatic power control mechanism is a closed-loop control mechanism. When a pickup head writes data to an optical disk with a laserbeam, the pickup head generates a driving current for controlling a laser diode (LD) to generate the laserbeam, and a front monitor diode (FMD) detects the power of laserbeam and generate an FMD output signal. The FMD output signal is sampled as references for adjusting the driving current of the laser diode. In FIG. 2, the laserbeam generated by the laser diode comprises multiple power levels, such as cooling power, erase power, write power, and over drive power, etc., in order to write data onto disk. Two power levels including for example a write power level and an erase power level are respectively sampled according to corresponding sample pulses for power level adjustment.
When the optical disk is a blu-ray disk, data density is increased. The laserbeam for writing data onto the blu-ray disk therefore has power levels that last for a shorter duration which is becoming smaller as recording speed getting higher. Referring to FIG. 3, a schematic diagram of signals generated by an automatic power control mechanism corresponding to a blu-ray disk is shown. There are two different writable area in a blu-ray disk, the data area and the APC area. APC area is utilized to perform automatic power control and data area is utilized to write the normal data. When a pickup head writes data (such us NRZ (Non-Return to Zero) signal in FIG. 3) to the blu-ray disk with a high recording speed, each power level of the laserbeam only lasts for a short duration. Because a front monitor diode requires a longer time period to appropriately generate a stable output signal, the FMD output signal does not converge to a real amplitude during the period for each power level and cannot be taken as feedback for correct automatic power control. The APC area is being set to solved this problem, but the APC area is relative smaller to the data area and the drive may not have enough time to get a stable write power in the beginning of the data writing. A method for automatic power control for an optical disk drive is therefore required.